I don't agree with any of that. So far in the new world luffy's 2 greatest accomplishments are defeating doflamingo and cracker. And defeating cracker, at least on hype alone, is the greater accomplishment between the two. And both fights suggest luffy is far from the level of either. Luffy did not win either fight on his own, he won them because of the spectacular support he had. Have luffy fight doflamingo without law or cracker without nami and the bulk of what he did against them is impossible. The victories luffy had over either of them were paper thing and that with all the support luffy could get. It took two runs of gear 4 to beat doflamingo and that was after law had ripped doflamingo's insides to shreds. Remove that and luffy would have definitely died in dressrosa. Against cracker luffy managed to hold his ground because nami made the cracker army useless. Which enabled luffy to eat it and manage his gear 4 balloon thing. For the suggestion that luffy is comparable to a yonko's top 3 commanders, let alone an admiral, to be reasonable luffy would have to be capable of defeating doflamingo or cracker without a speck of help. As in, without law to open use doflamingo's insides, without an army to keep doflamingo at bay while luffy recovers from gear 4 overuse and without nami to turn cracker's army into edible mush. And what makes you think luffy can hold his ground against mihawk? Mihawk is an end of manga character for zoro to beat, there is no way luffy right now has a chance against him. The most luffy could do against him is not die outright however if luffy fights him he is 100% dead.
Ultimately the main issue luffy has here is that marco is just a bad match up. Marco is able to instantly regenerate any damage he takes. He can't do it infinitely however that means that to defeat marco you have to either restrain him somehow or cause so much damage that he can't continue regenerating. Luffy has no means of restraining marco which means his only option is a battle of attrition. Which would be hard enough even if luffy could rag doll marco the same way as doflamingo. Which in itself is a stretch. When luffy uses gear 4 he has a time limit to win fights. Once his stamina and haki give out he is dead. If luffy couldn't beat a half stitched up doflamingo then he certainly has 0% chance against someone stronger than doflamingo who on top of everything heals from every blow he takes. Battles of attrition are not in luffy's benefit and marco is someone who can attrition the shit out of a fight.